1. Field of the Invention
The present invention relates to an insulated wire formed by applying an insulation film paint on a conductor and baking it. More particularly, the invention relates to an insulated wire preferably used for coils in electrical equipments such as rotary electric machines.
2. Description of Related Art
Usually, an insulated wire used for coils in rotary electric machines and transformers is constructed such that a single layer or a plurality of layers of insulation film is provided on the outer periphery of a conductor which is formed so as to have a cross-sectional shape (e.g. round or rectangle) to conform to the usage and shape of the coil. As rotary electric machines used in automobiles are required to generate higher output and to become compact and light-weight in recent years, insulated wires that enable high-density winding onto a core of a coil are required. Furthermore, for a coil created by welding the terminals of comparatively short insulated wires, insulated wires that will not be adversely affected as the result of being welded are required.
For an insulation film paint applicable to an insulation film for the insulated wire, JP-A Shou 58 (1983)-34828 (corresponding to WO 81/01568 and U.S. Pat. No. 4,258,155) discloses a resin composition formed by blending 5 to 95% by weight of polyamide-imide and 95 to 5% by weight of polyetherimide. According to JP-A Shou 58 (1983)-34828, a sheet formed by hardening the resin composition has mechanical characteristics equivalent to those of polyetherimide as well as chemical resistance and heat resistance properties equivalent to those of polyamide-imide.
Furthermore, JP-A 2000-235818 also discloses an insulated wire in which on a conductor there is formed an insulation layer made of a resin composition, e.g. polyamide-imide or the like, having the strength to adhere to the conductor of 30 g/mm or more and a glass transition temperature (Tg) of 250° C. or more; and thereon there is formed another insulation layer that is a mixture of a resin composition, e.g. polyamide-imide or the like, having Tg of 250° C. or more and another resin composition, e.g. polyetherimide, polyether sulfone or the like, having Tg of 140° C. or more, and whose breaking elongation is 40% or more. The insulated wire described in JP-A 2000-235818 seems to have an insulation film having excellent flexibility and processing resistance that will not cause cracks thereon even though the film is subject to a severe winding process or a severe rolling process, and the insulation film also seems to have heat resistance equivalent to that of polyamide-imide.
Furthermore, JP-A 2001-155551 discloses an insulated wire created such that on a conductor there is formed
(a) a first insulation layer substantially composed of polyamide-imide and/or polyimide, and
thereon there is formed and laminated
(b) a second insulation layer formed by blending polyamide-imide A with thermoplastic resin B (polyetherimide, polyether sulfone or the like) having a glass transition temperature of 140° C. or more at a weight ratio A/B of 70/30 to 30/70, wherein: the ratio of thickness T1 of the first insulation layer to thickness T2 of the second insulation layer (T1/T2) is 5/95 to 40/60; and the amount of residual solvent is 0.05% by weight or less of the total amount of insulation film. The insulated wire described in JP-A 2001-155551 seems to have excellent processing resistance which will not cause damage to the film even if the film is subject to a severe rolling process or a severe winding process, also having high heat resistance equivalent to that of polyamide-imide, and further having excellent junction characteristics that prevent the foaming of the insulation film around the joint area due to heat during the process of joining the terminals of insulated wires as well as preventing the elongation of the discolored area.
Due to the recent requirements for smaller size, higher performance, and energy conservation of electric equipment, the application of inverter control in rotary electric machines is becoming more and more popular. Also, to meet the demand, inverter control is increasingly executed by a higher voltage and larger current (greater electric power). In that case, there is a problem in that high inverter surge voltage generated by inverter control adversely affects the insulation system of the coil in a rotary electric machine.
In order to prevent deterioration of the insulation film due to inverter surge voltage, it is necessary to suppress the generation of partial discharge in the insulation film, that is, it is necessary to make partial-discharge start voltage in the insulation film high. To do so, effective known methods are, for example, a method of increasing the thickness of the insulation film, and a method of decreasing the dielectric constant of the insulation film by the use of fluoric polyimide resin.
On the other hand, with the achievement of higher efficiency of electric equipment, improvement of the space factor of the insulated wire is further required. That is, a further increase in partial-discharge start voltage (partial-discharge start voltage of at least 900 Vp) is required without increasing the thickness of the insulation film (with a thickness of approximately 45 μm).
However, when forming an insulation film by the use of fluoric polyimide resin, there is a problem in that weak adhesion between the insulation film and a conductor is prone to cause peeling, resulting in the occurrence of insulation breakdown.
On the other hand, when a resin composition described in JP-A Shou 58 (1983)-34828 is used for the insulation film on an enameled wire, since the temperature at which the polyetherimide component softens is low, there is a problem in that if the wire is subject to a temporarily high temperature (e.g. rotary electric machine overload operating condition or the like), a short-circuit occurs. Furthermore, in the insulated wires disclosed in JP-A 2000-235818 and JP-A 2001-155551, there is a possibility that a malfunction may occur due to the temperature at which a polyetherimide component softens is low, and also the partial-discharge start voltage is not high enough.